For example, a high intensity discharge (HID) lamp is used in a projector of an optical apparatus for displaying an image, such as a liquid crystal projector or a DLP (trademark) projector. In such a projector, light is split into three primary color components of red (R), green (G), and blue (B) by using a dichroic prism or the like, and images of respective three primary colors are generated by using spatial modulation elements provided for the respective three primary colors. The paths of the generated three primary color images are combined by using a dichroic prism or the like, so as to display a full color image on a screen or display.
In another known type of projector, light emitted from a light source is passed through a rotating dynamic color filter having transmission color wheel having a three primary color (R, G, and B) filter, thereby sequentially generating light rays of the three primary colors. In synchronization with the generation of the light rays, a spatial modulation device is controlled so as to sequentially generate images of respective three primary colors thereby displaying a color image.
In such a discharge lamp lighting apparatus in which a discharge lamp is lighted, a dielectric breakdown is caused in a discharge space at initiation of lighting, by applying high voltage to the discharge lamp, while non-load open voltage is impressed thereto. The state of the discharge caused by the application of the high voltage changes from glow discharge to arc discharge, so that stable lighting is realized. The lamp discharge voltage which was low, for example, about 10 V after the transition to the arc discharge, rises, as the temperature of the lamp rises. And, at a steady state of lighting, the lamp voltage is stabilized at fixed voltage.
Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply device so as to be suitable for the lamp discharge voltage so that lamp current required in order to realize predetermined power to be applied to the lamp can be outputted.
Moreover, the lamp voltage, i.e., the output voltage of the converter, is detected, and based on this information, the target electric current is determined by a value obtained by truncating a value obtained by dividing the target electric power by the detected voltage.
As discharge lamp driving methods, there are a direct-current drive method in which the lamp is turned on by a converter, and an alternating current driving method in which periodic polarity inversions are carried out by further providing an inverter in the output side of the converter.
In the case of the direct-current drive system, since the flux of light from a lamp does not change like the alternating current method (time-wise), either, it is advantageous that the method can be similarly applied to the both types of projectors which are described above.
On the other hand, in the case of the alternating current driving method, while growth or wear of the electrode(s) of the discharge lamp can be controlled by using the flexibility of polarity-inversion frequency which does not exist in the direct-current driving method, there is also a disadvantage resulting from the polarity inversions, such as an instantaneous light-out at the time of polarity inversions, or overshoot, etc. thereby affecting a display image adversely.
In order to obtain high color-reproduction performance of a display image, spectrum distribution of a light source lamp and adjustment of the conversion form of the sequential color light flux produced by using the dynamic color filter are important. In the case of the color wheel, color-reproduction performance or desired color-reproduction performance can be improved by setting up angle distribution of the respective color areas R, G, B (and W (white) if necessary), i.e., the rate of the time in which each color per rotation is transmitted, according to the spectrum of the lamp.
For example, when the lamp in which the R component is not sufficient is used, the R component transmission area is enlarged. That is, it is effective to make the rate of the time in which the R component is transmitted, longer than those of the other colors. However, in order to obtain a desired color-reproduction performance by such a method, for example, in a DLP type projector, the brightness for every color of each pixel of a display image is controlled by the duty cycle ratio of operation for each pixel of a space modulation element. Therefore, in the color component whose transmission time rate is reduced, there is a problem that fine control of the tone of a pixel cannot be easily carried out.
In order to solve the problem, in Japanese Laid Open Patent No. 08-505031, an image projection apparatus having a light source drive control unit in which output power of the light source is changed synchronizing with a color of the optical beam given by an output of a color change unit, is proposed. However, the concrete structure of such a light source drive control unit is not shown in the reference.
Similarly, in Japanese Laid Open Patent No. 2004-526992, a color-display apparatus in which electric power corresponding to colors is supplied, is proposed. In this reference, an electric wiring block diagram of an arc discharge lamp stabilization circuit capable of changing the electric power supplied to the lamp at high speed is disclosed. However, the circuit can set up only two power supply levels. Thus, the circuit cannot set up the electric power supplied to the lamp according to three colors R, G, and B or four colors R, G, B and W.
Moreover, in such a circuit, when the balance of the power level to be set up is changed, it is necessary to replace resistors of the circuit with those having different resistance.
Thus, in order to solve the problem, it has been known that high-speed modulation of the brightness of light from the light source which is synchronized with a conversion operation of the sequential color light flux carried out by dynamic color filter is useful, but high speed modulation of the brightness having arbitrary property at a low cost has not been realized.
On the other hand, Japanese Laid Open Patent No. 2005-267933 discloses that a lamp current detection signal conversion circuit is provided, wherein the gain thereof can be changed by switching corresponding to two or more bits modulation signal (M0, M1, . . . ). According to this structure of the discharge lamp lighting apparatus, the brightness of light from the discharge lamp light source is modulated (adjusted) with an arbitrary waveform at high speed. If this technology is simply used so as to realize the high-speed modulation of the brightness of light from the light source which is synchronized with a conversion operation of the sequential color light flux which is carried out by using a dynamic color filter, a circuit arrangement of a modulation signal generation circuit (Un) which generates the modulation signals (M0, M1, . . . ) can be shown in FIG. 15.
A signal which is generated based on an operation of a dynamic color filter (Of) in order to control timing of modulation condition change, and which is inputted from the outside of the discharge lamp lighting apparatus, for example, in case that in a color wheel type, a modulation switching timing signal (So) which is generated like a pulse at every color transition accompanied by rotation, is inputted in a clock pulse input terminal of a counter (Wc) which is formed by using a general-purpose IC, such as SN74HC161. A signal which is generated based on an operation of the dynamic color filter (Of) in order to identify the phase in a sequential color cycle, and which is inputted from the outside of the discharge lamp lighting apparatus, for example, in case that in a color wheel type, a modulation cycle initialization signal (Sop) which is generated like a pulse once per rotation and which is superposed on a particular modulation switching timing signal (So) is inputted in a clear input terminal of the counter (Wc). According to the structure, color information of the dynamic color filter (Of) which appears at that time can be identified by using a color code signal (WeA, WeB) which is a 2-bit count value output of the counter (Wc). Here, it is an assumption that the dynamic color filter (Of) has four colors areas (R, G, B, and W).
The color code signals (WeA, WeB) are inputted into a decoder (Wd) which is formed by using a general-purpose IC, such as SN74HC139, and only one corresponding to the color appearing at the time, of the four color information selection signals (Wf0, Wf1, Wf2, Wf3) which are outputs thereof is activated. The color information selection signals (Wf0, Wf1, Wf2, Wf3) are inputted into control input terminals of tri-state gates (Wg0, Wg1, Wg2, Wg3) which are respectively provided corresponding to these signals, wherein, for example, the tri-state gates are formed by using a general-purpose IC, such as SN74HC244.
The switch arrays (Ws0, Ws1, Ws2, Ws3), each of which comprises four switches, for setting up connection/disconnection to the ground are connected to input data terminals of the tri-state gates (Wg0, Wg1, Wg2, Wg3), respectively, and are pulled up with a resistor array (Wra) which comprises sixteen resistors to a power supply terminal (Wtp). In addition, the reason that the switch arrays (Ws0, Ws1, Ws2, and Ws3) comprise the four switches respectively, is that, the modulation signals (M0, M1, M2, M3) comprises 4-bits. Thus, it is possible to increase or reduce the number of the switches, if necessary, according to resolution required for modulation. According to the structure, since only one of input data corresponding to the color information which appears at that time, among the tri-state gates (Wg0, Wg1, Wg2, Wg3) is outputted to an output-data terminal, and the other output-data terminals for the other input data are maintained at a high impedance, it is possible to generate the modulation signals (M0, M1, M2, M3) by sending the output data of the bits to which the tri-state gates (Wg0, Wg1, Wg2, Wg3) correspond, respectively.
Namely, in the modulation signal generation circuit (Un) shown in FIG. 15, according to the modulation switching timing signal (So) and the modulation cycle initialization signal (Sop) which are generated based on an operation of the dynamic color filter (Of), a setting of the switch connection/disconnection state of one of the switch arrays (Ws0, Ws1, Ws2, Ws3) which corresponds to the color information appearing in the dynamic color filter (Of) at the time, is selected, so that the modulation signals (M0, M1, M2, M3) as digital value can be generated. Specifically, when the modulation switching timing signal (So) is received, the modulation signals (M0, M1, M2, M3) are generated according to a setting of the switch array (Ws1). After that, when the modulation switching timing signal (So) is received, the modulation signals (M0, M1, M2, M3) are generated according to a setting of the switch array (Ws1). After that, when the modulation switching timing signal (So) is received, the modulation signals (M0, M1, M2, M3) are generated according to a setting of the switch array (Ws2). Further, after that, when the modulation switching timing signal (So) is received, the modulation signals (M0, M1, M2, M3) are generated according to a setting of the switch array (Ws3), respectively. After that, this operation is repeated. And theoretically it is possible to realize a discharge lamp lighting apparatus capable of the high-speed modulation of the brightness of the light source which is synchronized with a conversion operation of the sequential color flux of light using a dynamic color filter which is carried out by using the above-mentioned modulation signal generation circuit (Un) as a modulation signal generation circuit (Un) in the technology disclosed in Japanese Laid Open Patent 2005-267933.
However, the modulation signal generation circuit (Un) shown in FIG. 15 has disadvantages. First of all, the number of circuit elements is large, so that the cost thereof tends to be high. In addition, since the circuit substrate occupies a large space, it is disadvantageous in terms of miniaturization of the discharge lamp lighting apparatus.
Secondly, since the modulation signals (M0, M1, M2, M3) according to the settings of the switch arrays (Ws0, Ws1, Ws2, Ws3) is repeatedly generated as described above, when change of the modulation pattern is required, it is necessary to change the connection/disconnection pattern of the switch arrays (Ws0, Ws1, Ws2, Ws3). That is, it is practically impossible to change the modulation pattern after the projector is shipped from the factory.
When making the patterns of a modulation waveform, i.e., modulation, changeable by an operation of a user, it is possible to mount two or more sets of switch arrays (Ws0, Ws1, Ws2, Ws3) each of which comprising four switches, and to choose one of them. Although it is necessary to eliminate the switch arrays (Ws0, Ws1, Ws2, Ws3), and, for example, instead of that, it is necessary to supply sixteen input signals of the tri-state gate (Wg0, Wg1, Wg2, Wg3) from output ports of a microprocessor etc.
In any case, there is a problem that a circuit becomes large-scale. Especially in the case of the latter, it is very difficult to assign the sixteen output ports to the microprocessor.
In order to avoid this, instead of the switch arrays (Ws0, Ws1, Ws2, Ws3), for example, a shift register with a parallel output, formed by using a general-purpose IC, such as SN74HC164, is mounted, so as to solve the assignment problem of the output ports of the microprocessor by configuring it so that one of the output ports of the microprocessor may be assigned to a data bit and one of them may be assigned to a shift clock. However, since the disadvantages about high cost and the miniaturization because of the number of the parts of circuit elements cannot be eliminated, it is necessary to adopt an IC etc., for exclusive use therefor, but this method can be applied only in case of mass production.